1. Field of the Invention
The present invention relates generally to acoustooptic devices such as acoustooptic modulators, acoustooptic deflectors, acoustooptic filters, acoustooptic frequency shifters, etc. Particularly, the present invention relates to ultraviolet acoustooptic devices, such as acoustooptic modulators, acoustooptic deflectors, acoustooptic filters, acoustooptic frequency shifters, etc. that utilize ultraviolet light with a wavelength of 380 nm or shorter, and to optical imaging apparatuses using the same.
2. Related Background Art
Conventionally, a TeO2 crystal or a PbMoO4 crystal has been used for acoustooptic media for visible light emitted from an argon laser or a helium-neon laser. On the other hand, recently, it has been studied to combine an acoustooptic device with various types of ultraviolet sources that emit lights with shorter wavelengths, i.e. ultraviolet light, such as, for example, a YAG laser emitting third and fourth harmonics. Herein, examples of the acoustooptic device include acoustooptic modulators, acoustooptic deflectors, acoustooptic filters, acoustooptic frequency shifters, etc.
Quartz glass, a quartz crystal, a KH2PO4 (KDP) crystal, or the like has been used as a medium of a conventional acoustooptic modulator for lights within the ultraviolet range (see Non-Patent Reference 1: Proceeding IEEE Ultrasonic Sympo, Vol. 1998, pp. 1289-1292 (1998); and Non-Patent Reference 2: Proceeding of the IEEE, Vol. 61, No. 8, pp. 1073-1092 (1973)).
On the other hand, an acoustooptic device in which a LiNbO3 (hereinafter referred to as “LN”) crystal is used also has been reported, but it utilizes light with a wavelength of 400 nm or longer (Non-Patent Reference 2 mentioned above).
In the above-mentioned conventional acoustooptic device in which a PbMoO4 crystal is used, since the absorption edge wavelength is around 410 nm in the PbMoO4 crystal, light with a wavelength of 380 nm or shorter is not transmitted therethrough. On the other hand, in the conventional acoustooptic device in which a TeO2 crystal is used, although the absorption edge wavelength is around 330 nm in the TeO2 crystal, it is not suitable for the use in which high power is used as in the embodiments of the present invention.
Furthermore, the acoustooptic device in which quartz glass, a quartz crystal, or a KDP crystal is used delivers poor acoustooptic performance, requires a large radio-frequency power source for driving the device, and has to be water-cooled to control the heat generated therein. In the acoustooptic device in which the KDP crystal is used, it is difficult to have a moisture resistant structure since the KDP crystal is a water-soluble crystal. Moreover, since the quartz crystal is a hard crystal, it takes a considerable time for processing it when it is used as an acoustooptic medium.
It has been conceived that the acoustooptic device in which a LN crystal is used is not suitable as an acoustooptic device that handles light with a short wavelength due to optical damage and laser damage caused therein.
In this connection, the “optical damage” denotes a state where space charge is excited by light and then an electric field is generated by the space charge and thereby changes the refractive index of the crystal. Light passing through a medium with the optical damage caused therein passes through a region with its refractive index changed locally. As a result, a considerably deteriorated beam shape is observed because of the Pockels effect.
Furthermore, the “laser damage” denotes a state where the crystal is damaged physically at its surface or inner part by the influence of a strong laser beam. For instance, a strong laser beam causes ablation at the surface of a crystal to form a concave portion.
In an optical imaging apparatus using a conventional ultraviolet acoustooptic device, there have been problems in that its acoustooptic medium has to be provided with moisture resistance, heat has to be dissipated by water-cooling, and a large driving circuit is required.